Adhesions are unwanted tissue growths occurring between layers of adjacent bodily tissue or between tissues and internal organs. Adhesions commonly form during the healing which follows surgical procedures, and when present, adhesions can prevent the normal motions of those tissues and organs with respect to their neighboring structures.
The medical and scientific communities have studied ways of reducing the formation of post-surgical adhesions by the use of high molecular weight carboxyl-containing biopolymers. These biopolymers can form hydrated gels which act as physical barriers to separate tissues from each other during healing, so that adhesions between normally adjacent structues do not form. After healing has substantially completed, the barrier is no longer needed, and should be eliminated from the body to permit more normal function of the affected tissues.
Several different types of biopolymers have been used for this purpose. For example, Balazs et al., U.S. Pat. No. 4,141,973 discloses the use of a hyaluronic acid (HA) fraction for the prevention of adhesions. However, because HA is relatively soluble and readily degraded in vivo, it has a relatively short half-life in vivo of 1 to 3 days, which limits its efficacy as an adhesion preventative.
Methyl cellulose and methyl cellulose derivatives are also known to reduce the formation of adhesions and scarring that may develop following surgery. (Thomas E. Elkins, et al., Adhesion Prevention by Solutions of Sodium Carboxymethylcellulose in the Rat, Part I, Fertility and Sterility, Vol. 41, No. 6, June 1984; Thomas E. Elkins, M. D. et al., Adhesion Prevention by Solutions of Sodium Carboxymethylcellulose in the Rat, Part II, Fertility and Sterility, Vol. 41. No. 6, June 1984. However, these solutions are rapidly reabsorbed by the body and disappear from the surgical site.
In addition to solutions of carboxyl-containing biopolymers, solutions of polyethers can also decrease the incidence of post-surgical adhesions. Pennell et al., U.S. Pat. No. 4,993,585 describes the use of polyethylene oxide in solutions of up to 15% to decrease formation of post-surgical adhesions. Pennell et al., U.S. Pat. No. 5,156,839 describes the use of mixtures of carboxymethylcellulose up to about 2.5% by weight, and polyethylene oxide, in concentrations of up to about 0.5% by weight in physiologically acceptable, pH neutral mixtures. Because of the neutral pH, these materials do not form association complexes, and thus, being soluble, are cleared from the body within a short period of time.
The above-described solutions have several disadvantages. First, they have short biological residence times and therefore may not remain at the site of repair for sufficiently long times to have the desired anti-adhesion effects.
Although the methods of manufacture of certain carboxypolysaccharide-containing membranes have been described, the membranes are poorly suited for use to prevent adhesions. Butler, U.S. Pat. No. 3,064,313 describes the manufacture of films made of 100% carboxymethylcellulose (CMC) with a degree of substitution of 0.5 and below, made insoluble by acidifying the solution to pH of between 3 and 5, and then drying the mixture at 70.degree. C. to create a film. These films were not designed to be used as anti-adhesion barriers. Anderson, U.S. Pat. No. 3,328,259 describes making films of 100% carboxymethylcellulose and polyethylene oxide, alkali metal salts, and a plasticizing agent for use as external bandages. These materials are rapidly soluble in plasma and water and thus would have a very short residence time as an intact film. Therefore, these compositions are not suitable for alleviating surgical adhesions.
Smith et al., U.S. Pat. No. 3,387,061 describes insoluble association complexes of carboxymethylcellulose and polyethylene oxide made by lowering the pH to below 3.5 and preferably below 3.0, and then drying and baking the resulting precipitate. These membranes were not designed for surgical use to alleviate adhesions. Such membranes are too insoluble, too stiff, and swell to little to be ideal for preventing post-surgical adhesions. In addition, their excessive acidity would cause tissue inflammation.
Burns et al., U.S. Pat. No. 5,017,229 describes water insoluble films made of hyaluronic acid, carboxymethyl cellulose, and a chemical cross-linking agent. Because of the covalent cross-linking with a carbodiimide, these films need extensive cleaning procedures to get rid of the excess cross-linking agent; and because they are made without a plasticizer, they are too stiff and brittle to be ideally suited for preventing adhesions--they do not readily conform to the shapes of tissues and organs of the body.
Thus, there have been few successful antiadhesion membranes. D. Wiseman reviews the state of the art of the field in Polymers for the Prevention of Surgical Adhesions, In: Polymeric Site-specific Pharmacotherapy, A. J. Domb, Ed., Wiley & Sons, (1994). A currently available antiadhesion gel is made of ionically cross-linked hyaluronic acid. Huang et al., U.S. Pat. No. 5,532,221. Cross-linking is created by the inclusion of polyvalent cations, such as ferric, aluminum or chromium salts. Unfortunately, hyaluronic acid (either from natural sources or bioengineered) is quite expensive. Therefore, the prior art discloses no membranes ideally suited to the variety of surgical uses of the instant invention. Thus, there are several objects of the instant invention.
A first object is to provide compositions and methods which reduce the incidence of adhesion formation during and after surgery. This includes the prevention of de novo adhesion formation in primary or secondary surgery.
An additional object is to prevent reformation of adhesions after a secondary procedure intended to eliminate the de novo adhesions which had formed after a primary procedure.
Another object is to provide an inexpensive antiadhesion membrane which remains intact at the surgical site during the initial stages of critical wound healing.
Yet another object of the invention is to provide an antiadhesion membrane which can hydrate quickly in a controlled fashion to form an intact hydrogel.
An additional object of the invention is to provide an antiadhesion membrane which is resorbable and completely eliminated from the body.
A further object of the invention is to provide an antiadhesion membrane which has good handling characteristics during a surgical procedure, is conformable to a tissue, pliable, strong, and easy to mold to tissue surfaces, and possesses sufficient bioadhesiveness to ensure secure placement at the surgical site until the likelihood of adhesion formation is minimized.
Yet another objective of the invention is to provide an antiadhesion membrane with desired properties with drugs incorporated into the membrane, so that the drug can be delivered locally over a period of time to the surgical site.
To achieve these objectives, the instant invention involves carefully controlling the properties of antiadhesion membranes by closely regulating the pH, amounts of carboxyl residues and polyether within the carboxypolysaccharide/polyether association complex, to closely control the degree of association between the polymers. By carefully controlling the degree of intermolecular binding and amount of polyether, we can closely vary the physical properties of the membranes and therefore can optimize the antiadhesion, bioadhesive, bioresorptive, and antithrombogenic properties of the membranes to achieve the desired therapeutic results.
Too much hydration can result in an irreversible transformation of the membrane to a "loose gel" which will not stay in place or will disintegrate. In addition, too much swelling can create too much hydrostatic pressure which could adversely affect tissue and organ function. The membrane must be physiologically acceptable, be soft, have the desired degree of bioresorbability, have the desired degree of antithrombogenicity, and must be biologically inert.